Display screen or portion thereof with a transitional graphical user interface



FIG. 1 is a front view of a display screen or portion thereof with a transitional graphical user interface, showing the new design;

FIG. 2 is a front view of a second image thereof;

FIG. 3 is a front view of a third image thereof;

FIG. 4 is a front view of a fourth image thereof;

FIG. 5 is a front view of a fifth image thereof;

FIG. 6 is a front view of a sixth image thereof;

FIG. 7 is a front view of a seventh image thereof;

FIG. 8 is a front view of an eighth image thereof;

FIG. 9 is a front view of a ninth image thereof;

FIG. 10 is a front view of a tenth image thereof;

FIG. 11 is a front view of an eleventh image thereof;

FIG. 12 is a front view of a twelfth image thereof;

FIG. 13 is a front view of a thirteenth image thereof;

FIG. 14 is a front view of a fourteenth image thereof;

FIG. 15 is a front view of a fifteenth image thereof;

FIG. 16 is a front view of a sixteenth image thereof; and,

FIG. 17 is a front view of a seventeenth image thereof.

The appearance of the image transitions sequentially between the images shown in FIGS. 1-17. The process or period in which an image transitions to another forms no part of the claimed design. The broken line showing of a display screen, text, and background is for illustrative purposes only and forms no part of the claimed design. 

CLAIM The ornamental design for a display screen or portion thereof with a transitional graphical user interface, as shown and described. 